This application claims the priority of German Patent Application Serial No. 101 27 238.3, filed May 28, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to an apparatus for ascertaining a cable motion for a hoist, in particular a pneumatically operated cable balancing hoist. The present invention also relates to a hoist having incorporated therein an apparatus for ascertaining a cable motion, and to a cable drum for use with a hoist and cooperation with an apparatus for ascertaining a cable motion.
German Pat. Nos. DE 1928683 and DE 2003837 describe each a pneumatic hoist for balancing the weight of a load. The hoist includes a cable drum in which a piston is arranged for movement in longitudinal direction. The cable drum is connected to a spindle in fixed rotative engagement, with the spindle extending in coaxial relationship to the cable drum and turning a nut which is securely fixed to the housing. As the piston is acted upon by gas pressure and displaced, the cable drum is caused to move with the piston and to rotate as the spindle turns in the nut. The cable drum is formed with a drum groove for a cable which is guided downwards from a run-off point to a load-carrying member. The pitch of the drum groove is so selected that the run-off point of the cable is always at the same spot of the housing.
It is also known in conjunction with a slip-free determination of the cable movement to arrange at one end of the cable drum a circular disk which is driven by the cable drum and provided with markings that evenly spaced from one another. As the cable drum rotates, the circular disk is caused to rotate and the markings move past a sensor of a sensor unit which detects the markings. Thus, the sensor unit provides an output signal commensurate with a rotation movement of the cable drum. This approach suffers shortcomings because the apparatus is complex and cumbersome.
It would therefore be desirable and advantageous to provide an improved apparatus for slip-free determination of a cable motion for a hoist, which obviates prior art shortcomings and which is compact and simple in structure while still being reliable in operation.
According to one aspect of the present invention, an apparatus for ascertaining a cable motion in longitudinal cable direction for a hoist of a type having a housing, and a cable drum which has an outer surface area, formed with a groove for receiving a cable, wherein the cable drum is rotatably supported in the housing and moveable in a longitudinal direction in such a way that a run-off point of the cable is at a same location, includes a sensor for detecting markings of the cable drum to deliver an output signal as the markings move relative to the sensor during rotation of the cable drum, wherein the groove defines a helical ridge in parallel relationship to the groove, wherein the markings are provided along the ridge, and wherein the sensor is secured to the housing in confronting relationship to the ridge.
The present invention resolves prior art problems by exploiting the ridge as substrate or carrier element for the markings so that the apparatus becomes rugged and has a simple construction because it is only required to arrange a sensor on the housing and to provide the markings on the ridge.
Generation of an output signal in a simple way may be realized by providing the markings in the form of single marking elements having defined length and moving conjointly with the cable drum.
According to another feature of the present invention, the markings may be arranged on a ferrite strip which carries the markings in the form of alternating magnetic and nonmagnetic regions of same length and same spaced-apart relationship. In this way, simple markings are provided which can be detected without contact. Suitably, the ferrite strip is supported on a steel band as substrate. A compact construction can be implemented when forming the ridge with a groove for receiving the steel substrate.
According to another feature of the present invention, the sensor may be a Hall sensor to realize a reliable and simple detection of the markings.